Circuits having negative resistance characteristics



Dec. 14, 1965 c. D. TODD 3,223,849

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CIRCUITS HAVING NEGATIVE RESISTANCE CHARACTERISTICS Filed Jan. 2, 1962 5SheetsSheet 5 United States Patent 3,223,849 CIRCUITS HAVING NEGATIVERESESTANCE CHARACTERISTICS Carl D. Todd, Costa Mesa, Califl, assignor toHughes Aircraft Company, Culver City, Calif., a corporation ofCalifornia Filed Jan. 2, 1962, Ser. No. 163,808 3 Claims. (Cl. 307-885)This invention relates to negative resistance circuits and particularlyto improved circuits or devices including semiconductor elements andexhibiting S-type negative resistance characteristics.

Customarily, negative resistance devices are divided into the majorclasses of N-type or current stable devices and S-type or voltage stabledevices. In a graph having current plotted on the X axis and voltageplotted on the Y axis, the curves representing the characteristics ofthe N-type and S-type devices respectively have the shape of the letterN or the letter S. Depending upon the particular result desired, one orthe other type or either type may be utilized. Some conventional S-typenegative resistance devices are the vacuum tube tetrode as used in thedynatron oscillator, the point contact transistor as used in the commonemitter configuration and the tunnel diode. In many instances, it isdesirable that the negative resistance be relatively linear over a widerange of voltage or current swing. Conventional negative resistancedevices have relatively poor linearity in the negative resistance regionwhich may cause undesirable operation in many applications. Also, manyconventional negative resistance devices do not have the property thatthe basic parameters may be easily varied according to desiredrequirements.

It is therefore an object of this invention to provide a negativeresistance device having an S-type voltagecurrent characteristic and anegative resistance that is relatively linear over a wide range ofvoltage or current.

It is a further object of this invention to provide an S-type negativeresistance device utilizing semiconductor elements and providing ahighly linear negative resistance over a wide range of voltage andcurrent.

It is another object of this invention to provide an improved negativeresistance circuit that may be utilized as a two terminal or a threeterminal device.

It is still another object of this invention to provide S-type negativeresistance devices having arrangements for varying or selecting desiredoperating characteristics.

Briefly, in accordance with the invention an emitter follower transistoris coupled to a common base transistor of the opposite type to form acurrent amplifier. The output terminals may be coupled to the electrodesof the common base transistor. Current is supplied to the base of theemitter follower transistor both from a voltage source through a firstresistor and from the output terminals through a feedback path includinga sec ond resistor so as to provide a negative resistance. In a threeterminal arrangement in accordance with the invention, a controlterminal is also coupled to the base of the emitter follower transistor.

The novel features of this invention, as well as the invention itself,both as to its organization and method of operation, will best beunderstood from the accompanying description taken in connection withthe accompanying drawings, in which like characters refer to like parts,and in which:

FIG. 1 is a schematic circuit diagram of a two terminal circuit inaccordance with this invention providing an S-type negative resistancecharacteristic;

7 3,223,849 Patented Dec. 14, 1965 ICC FIG. 2 is a graph of currentversus voltage for explaining the operation of the circuit of FIG. 1without the feedback arrangement;

FIG. 3 is a graph of current versus voltage for expaining the negativeresistance characteristics of the circuit of FIG. 1;

FIG. 4 schematically illustrates a circuit in accordance with theinvention for providing a control of the breakdown voltage of thenegative resistance circuit of FIG. 1;

FIG. 5 is a schematic circuit diagram of a second arrangement inaccordance with this invention for providing a control of the breakdownvoltage of the negative resistance circuit of FIG. 1;

FIG. 6 is a graph of terminal current versus terminal voltage forexplaining the controlled negative resistance characteristics developedby the circuit of FIG. 5;

FIG. 7 is a schematic circuit diagram of a three terrninal negativeresistance device in accordance with this invention;

FIG. 8 is a graph of terminal current versus terminal voltage forexplaining the controlled negative resistance characteristics of thecircuit of FIG. 7;

FIG. 9 is a schematic circuit diagram of a monostable circuit inaccordance with the invention;

FIG. 10 is a graph of current versus voltage for explaining theoperating characteristics of the monostable circuit of FIG. 9.

Referring to the basic circuit in accordance with the invention, whichis illustrated in FIG. 1, a current amplifier 12 having a current gain Ais provided coupled to first and second terminals 16 and 18 throughrespective leads 20 and 22. The terminals 16 and 18 may be consideredoutput terminals in the two terminal arrangement.

The lead 20 is also coupled to the current amplifier 12 by a feedbackresistor 24 and a lead 25, the resistor 24 having a value R The currentamplifier 12 includes an n-p-n type common base transistor 28 whichdrives the emitter of an emitter follower transistor 30 of the p-n-ptype. The collector of the transistor 28 is coupled to the lead 20through a common point 21 and the base is coupled to the referencepotential on the lead 22. The collector of the transistor 30 is coupledto the negative terminal of a source of potential such as a battery 32having a positive terminal coupled to the lead 22 and the base of thetransistor 30 is coupled to a junction point 36. A base resistor 38having a value R is coupled between the junction point 36 and thenegative terminal of the battery 32. The junction point 21 is coupledthrough a resistor 24, having a value R to a lead which is coupled tothe junction point 36. Currents I and I flow into the junction point 36respectively from the lead 25 and from the base of the transistor 30.The current I flows from the junction point 36 and may have asubstantially constant value. The current I flows into the terminal 16with the currents I and I flowing respectively from the junction point21 to the resistor 24 and to the collector of the transistor 28. The DC.(direct current) voltage developed across the battery 32 is representedby V and the voltage applied across the input terminals 16 and 18 isshown as an instantaneous voltage value designated as V which may beapplied from a source 44.

For explaining the operation of the circuit of FIG. 1, it will be firstassumed that the feedback path of the resistor 24 is nonexistent. Thus,the output terminals are effectively those of a common base transistorwith a constant emitter bias resulting in a terminal current versusterminal voltage characteristic shown by a curve 48 of FIG. 2. For aterminal voltage V which is slightly negative because of the saturationcondition of the tran- S sistor, the output resistance of the transistor28 or circuit terminal resistance is relatively low, that is, the curve48 has a relatively steep slope. As the voltage V changes to zero orpositive, the output resistance increases greatly for the transistor 28as that transistor goes out of saturation. The output resistance is thenequal to the reciprocal of h for the transistor 28, where h is thecommon emitter small-signal output admittance with the input A.C.(alternating current) open-circuited. When the terminal volt-age Vattempts to exceed the collector-to-fbase breakdown voltage of thetransistor '28 at 50, the output resistance again decreases sharply. Thecollector breakdown voltage at the high voltage V 'at 50 may result fromaval-anching within the transistor 28, as is well known in the art.

The further operation of the circuit of FIG. 1 will be explained withthe feedback resistor 24 connected into the circuit to form acharacteristic negative resistance curve 54 of FIG. 3. The currentflowing through the feedback resistor 24 in the region where thetransistor '28 is saturated or when V is negative or at zero volts isapproximately zero so that the curve 54 is substantially the same as thecurve 48 (FIG. 2) in that region. As V is increased the current I alsomust increase as the voltage at the junction point 36 remains relativelyconstant. The voltage at the junction point 36 remains relativelyconstant as long as the emitter-to-base diodes of the transistors 28 and30 are conducting As the terminal voltage V increases. the current Iincreases and the base current 1 decreases because the current I flowingthrough the resistor 38 is relatively constant. Thus the current I bucksor subtracts from the base current 1 The net change in base current I isa decrease equal to the change of current I When 1 decreases, thecollector current of the transistor 28 will decrease 'and due to the netcurrent gain in the two transistors, the decrease in current I into thecollector of the transistor 28 is greater than the initial increase ofcurrent I The net result is a decrease in the total terminal current Ifor the given increase in terminal voltage V This change represents anegative resistance. The curve 54 decreases to a valley current I atwhich the current I equals the current I and the base current 1 isreduced to a very small leakage current.

To further explain the circuit of FIG. 1 the operation will first beanalyzed by considering the current amplifier 12 of FIG. 1 having thegain A, as a composite element having zero input resistance and aninfinite output resist- When a small signal A.C. voltage V, is appliedto ance. the terminals 16 and 18, a current I must flow.

This current I is then amplified by A and the current I The totalterminal current I, is the sum of I and I and is current, and h is thecommon base current gain of the transistor 28 or ratio of collector toemitter current. Inserting Equation 5 into Equation 4 gives thefollowing expression for terminal resistance:

RTE R1 In the above analysis showing the negative resistance developedby the circuit of FIG. 1, the effect of the output resistance of thetransistor 28 (the h of the transistor 28) was neglected. As the emitterof the transistor 28 is supplied from a relatively high resistance, theoutput resistance is approximately equal to the reciprocal of h which isthe common-emitter small-signal output admittance of the transistor 28with the input A.C. opencircuited. The value of h is given by the slopeof the characteristic curve 48 of FIG. 2. For a terminal voltage at theterminals 16 and 18 between zero and the breakdown voltage V the outputresistance is normally very high. At a low negative value of V theoutput resistance is low as shown by curve 54 of FIG. 3 but increases.sharply even before a zero value of V is reached. Also at very highvalues of V the output resistance decreases sharply as the transistorbreaks down at V As the terminal voltage V is increased, the current Iincreases and the base current 1 of the transistor 30 decreases as wellas the emitter current of the transistor 28. A level is reached as V isincreased where the operating bias currents of the transistors 28 and 30is so low that the current gain of both transistors approaches zero withthe transistor 30' having the greatest effect. The result as may be seenfrom Equation 6 is to increase the magnitude of R The terminalresistance as seen by Equation 6 will become infinite as V increases toreverse bias the transistor 30 and then approach the value of R Also,when the emitter-toabase junction of the transistor 30 is reverse biasedas shown at the portion 55 of the curve 54, the terminal resistance atthe terminals 16 and 18 becomes equal to the sum of R and R Thisterminal resistance is true only in the region of the portion 55 beforethe emitter-to-base of the transistor 28 breaks down.

When the terminal voltage V increases to the breakdown voltage V of thetransistor 28, the h increases sharply and the net value of the terminalresistance R reaches a low positive value as shown by the slope of theright hand portion of the curve 54. It is to be noted that the valleypoint I Where R is infinite 'may be determined by the decrease in gainof the transistor 30 or in some arrangements, as .will be discussedsubsequently, by the collector breakdown voltage of the transistor 28.The curve 54 in the negative resistance region has a slope l/R where Ris the negative resistance at the terminals 16 and 18. The linearity ofthe negative resistance portion of the curve 54 is primarily dependentupon the variation of k of the transistor 30 with changes of operatingcurrent level. The negative resistance has been found to be highlylinear in the circuits in accordance with this invention.

To explain the design flexibility of the circuits in accordance withthis invention some of the design factors will be considered. The mostimportant parameter for A.C. consideration is the magnitude of theterminal resistance R in the negative resistance region. Parameters ofinterest in switching and in relaxation circuits, as well as in biasingconsiderations, are primarily the currents and voltages defining thepeak and valley points and the voltage V at which the low terminalresistance is reached. For purposes of design it may be assumed that thevoltage V of the battery 32 of FIG. '1 is constant and fixed and thevalues of 11 and h of the transistor 30 are fixed. The characteristic his the static D.C. forward current transfer ratio for the common emitterconfiguration of a transistor. The parameters h and h and [1 of thetransistor 28 may be neglected when considering the negative resistanceregion. As is well known, the parameter h is the static D.C. forwardcurrent transfer ratio for the common base configuration and h is thecommon base small signal output admittance with the input of thetransistor A.C. open-circuited.

When designing for a desired value of negative resistance R the value ofthe resistance R may be determined Once the value of R is determined,the value of R may be chosen either to give the desired peak current Ior the necessary valley voltage V If a specific value of peak current Iis required, the value of R may be determined from the followingrelationship:

V =R I If design requirements make it necessary to design for a specificvalley voltage, the value of R may be determined from the followingequation:

In the design of the circuits in accordance with the invention, if thevoltage V of the battery 32 may be selected, the value should be largewith respect to the sum of the emitter-to-base diode drops of thetransistors 28 and 30 in order to decrease the effect of temperature onthe parameters V and V If this condition is not met selecting a largevalue of V the peak current I will increase as the temperature rises. Itis to be noted that I is only affected a small amount by changes of 11Because the breakdown voltage V of FIG. 3 1s relatively high, anarrangement is provided in accordance with this invention forcontrolling the breakdown voltage. By lowering the breakdown voltage,excessive power dissipation in the transistor 28 is prevented whenoperating at high current levels. The arrangement of FIG. 4 1ncludes abreakdown diode 60 having an anode coupled to ground which may be thesame reference level as the lead 22. The breakdown diode 68 may be adiode having Zener or avalanche breakdown characteristics for exampleThe cathode of the breakdown diode 60 is coupled through a resistor 62to the input lead 20. The arrangement of FIG. 5 includes a source ofclamping voltage such as a battery 66 having a negative terminal coupledto ground and having a positive terminal coupled to the cathode of aunidirectional device such as a diode 68. The anode of the diode 68 iscoupled through a resistor 70 to the input lead 20.

In operation the breakdown diode 60 of FIG. 4 is selected to providebreakdown at a voltage V before the breakdown of the transistor 28 asshown by curves 70, 72 and 74 of FIG. 6. It is to be noted that thecurves 7!), 72 and 74 represent operation with the resistors 62 and 70made to have zero value. The clamping voltage of the battery 66 in thearrangement of FIG. 5 may be selected to provide the desired breakdownpoint as shown by the curves 70, 72 and 74, for example. As the voltageV increases to V V or V either the diode 60 of FIG. 4 breaks down or thediode 68 of FIG. 5 is biased into conduction.

The resistor 62 in series with the breakdown diode 60 or the resistor 70 in series with the diode 68 can be varied to obtain additionalcharacteristics. Varying the value of the resistors 62 and adds to theA.C. resistance of the composite circuit to provide characteristics ofdotted curves 76, 78 or 80, respectively, resulting from increasing thevalue of the resistors.

Another arrangement in accordance with this invention is by coupling abreakdown diode 82 in series with the resistor 24 as shown in FIG. 4.The breakdown diode 82 may be a diode having Zener or avalanchebreakdown characteristics, for example. As a result, the peak voltage Vas shown by a curve 83 in FIG. 3 may be increased to any desired level.The peak voltage V will be approximately equal to the selected breakdownvoltage of the diode 82.

Another circuit in accordance with this invention utilizes the base ofthe emitter follower transistor 30 as a third terminal as shown in FIG.7. A terminal 86 which may be coupled to a current source 88 is coupledto the base of the transistor 30. The current source 88 provides aselected current to the base of the transistor 30 to vary thecharacteristics as shown by curves 90, 92, 94, 96 and 98 of FIG. 8. Apositive current bias, when current flows in the direction of an arrow89 from the variable current source 88, decreases the peak current Ifrom the value obtained when I has a zero value (curve 94) to the valuesof the curves such as 96 and 98. A negative current applied from thesource 88 so that current flows in a direction opposite the source 88 sothat current flows in a direction opposite to the arrow 89 increases thepeak current 1;. as shown by the curves 92 and 90. It is to be notedthat in the arrangement of FIG. 7, the negative resistance slopes of thecurves such as 92 and 96 are the same. This constant negative resistanceslope results because the current gain of the transistors 28 and '30 andthe value of the feedback resistor 24 have remained substantiallyconstant. The breakdown voltage V is independent of the emitter followertransistor 30 and the voltage V does not change with current applied tothe base of the transistor 30.

Another arrangement in accordance with this invention to generate thefamily of curves of FIG. 8 is to vary the value of the supply voltage Vof the battery 32 or by varying the value of the resistor 38.

It is to be noted that the types of transistors shown are to illustratethe invention and that opposite type transistors may be utilized inaccordance with this invention with the biasing voltages appropriatelychanged in a conventional manner.

The composite negative resistance circuits described above may beutilized in a variety of applications such as oscillators, pulseamplifiers, Q multipliers and switches, for example. The ability tochoose the parameters of the S-type negative resistance circuits allowsvery wide use thereof. In FIG. 9 the composite negative resistancecircuit is arranged as a monostable multivibrator in accordance withthis invention. The breakdown diode 60 is coupled between ground and thecollector of the transistor 28 to select a breakdown voltage V as shownin FIG. 10. The junction point 21 is coupled through a load resistor 102having a value R and an inductor 104 to a positive source of potential+V such as a terminal 106 for providing a constant current source. Apositive trigger signal or voltage of a waveform 108 may be applied to aterminal 112 through a switching diode 114 to the junction point 21 toform an output voltage pulse of a waveform 118. Also, the circuit may beconsidered as a three terminal device as previously discussed, and thepositive trigger pulse of the waveform 188 may be applied to a terminal122 and through a coupling capacitor 124 to the base of the transistor30.

In operation, a curve 128 of FIG. 10 shows the negative resistancecharacteristic of the circuit of FIG. 9. The current and voltagenormally applied to the output terminal 16 is at a stable point 131 asdetermined by a load line l/R In response to a trigger signal of thewaveform 108 applied either to the terminals 112 or 122, the peakcurrent 1,: is exceeded and the circuit changes state through thenegative resistance region along a path such as 134 to the breakdownvoltage V as determined by the diode 60 breaking down. As the triggersignal of the waveform 108 has a relatively short duration, the circuitchanges state along the curve 128 to a very low terminal current I whichdecreases to a value equal to I and then switching action occurs along apath 130 to a negative value of V and back to the point 131. Because ofthe clamping action of the breakdown diode 60, the output pulse of thewaveform 118 has a relatively flat top portion. When the circuit istriggered at the terminal 112, a relatively low power is requiredbecause of the circurt gain.

Thus, there has been described a composite circuit exhibiting an S-typenegative resistance characteristic utilizing in its simplest form, onlytwo resistors and two transistors. Because of the arrangement of theelements the negative resistance is relatively linear over a wide rangeof voltage or current swing. The composite circuit may be widely usedbecause of flexibility of design and modifications of the circuitcharacteristics.

What is claimed is:

1. A circuit for providing a current-voltage characteristic having alinear negative resistance portion of preselected magnitude betweenfirst and second positive resistance portions comprising: first andsecond terminals between which said current-voltage characteristic isprovided; a source of potential having one of its terminals connected tosaid second terminal; first and second transistors of oppositeconductivity types each having an emit ter, a collector and a base; thecollector and base of said first transistor being directly connected tosaid first and second terminals, respectively; the emitter of said firsttransistor being directly connected to the emitter of said secondtransistor; the collector of said second transistor being directlyconnected to the other terminal of said source of potential; resistancemeans connected between the base of said second transistor and saidother terminal of said source of potential for providing a substantiallyconstant current flow therebetween; and a feedback resistor connectedbetween the collector of said first transistor and the base of saidsecond transistor for decreasing the base current of said secondtransistor in response to an increase in voltage between said first andsecond terminals; said feedback resistor having a resistance value equalin magnitude to the common emitter current gain of said secondtransistor times said preselected magnitude of said linear negativeresistance.

2. A circuit for providing a current-voltage characteristic having alinear negative resistance portion of preselected magnitude betweenfirst and second positive resistance portions, said negative resistanceportion intersecting said first positive resistance portion at apreselected peak current and intersecting said second positiveresistance portion at a valley voltage, comprising: first and secondterminals between which said current-voltage characteristic is provided;a source of potential having one of its terminals connected to saidsecond teminal; first and second transistors of opposite conductivitytypes each having an emitter, a collector and a base; the collector andbase of said first transistor being directly connected to said first andsecond terminals, respectively; the emitter of said first transistorbeing directly connected to the emitter of said second transistor; thecollector of said second transistor being directly connected to theother terminal of said source of potential; a feedback resistorconnected between the collector of said first transistor and the base ofsaid second transistor for decreasing the base current of said secondtransistor in response to an increase in voltage between said first andsecond terminals, said feedback resistor having a resistance value Rgiven y 1= re n Where h is the common emitter current gain of saidsecond transistor and R is said preselected magnitude of said linearnegative resistance; a second resistor connected between the base ofsaid second transistor and said other terminal of said source ofpotential, said second resistor having a resistance value R given by[RF- 1P 1+hFE, where V is the static D.C. value of the base-to-emittervoltage of said second transistor, V is the instantaneous total value ofthe voltage between the emitter and base of said first transistor, V isthe voltage provided by said source of potential, hFEg is the staticD.C. forward current transfer ratio of said second transistor, and Ip issaid preselected peak current.

3. A circuit for providing a current-voltage characteristic having alinear negative resistance portion of preselected magnitude betweenfirst and second positive resistance portions, said negative resistanceportion intersecting said first positive resistance portion at a peakcurrent and intersecting said second positive resistance portion at apreselected valley voltage, comprising: first and second terminalsbetween which said current-voltage characteristic is provided; a sourceof potential having one of its terminals connected to said secondterminal; first and second transistors of opposite conductivity typeseach having an emitter, a collector and a base; the collector and baseof said first transistor being directly connected to said first andsecond terminals, respectively; the emitter of said transistor beingdirectly connected to the emitter of said second transistor; thecollector of said second transistor being directly connected to theother terminal of said source of potential; a feedback resistorconnected between the collector of said first transistor and the base ofsaid second transistor for decreasing the base current of said secondtransistor in response to an increase in voltage between said first andsecond terminals, said feedback resistor having a resistance value Rgiven by l feg n where h is the common emitter current gain of saidsecond transistor and R is said preselected magnitude of said linearnegative resistance; a second resistor connected between the base ofsaid second transistor and said other terminal of said source ofpotential, said second resistor having a resistance value R given by 2(VBE2+VEB1) (1 FE n where V is the static D.C. value of thebase-to-emitter voltage of said second transistor, V is theinstantaneous total value of the voltage between the emitter and base ofsaid first transistor, V is the voltage provided by said source ofpotential, h is the static D.C. forward current transfer ratio of saidsecond transistor, and VV is said preselected valley voltage.

References Cited by the Examiner UNITED STATES PATENTS 2,588,925 3/1952Hecht 330-112 2,879,412 3/1959 Hoge et a1. 307-885 2,885,495 5/1959Sziklai et al. 330-20 2,912,654 11/1959 Hansen 330-20 2,943,282 6/1960Pfietfner 333- 2,986,651 5/1961 Schayes 307-885 ther references onfollowing page) 9 10 UNITED STATES PATENTS OTHER REFERENCES 7/ 1961Floyd 307-88.5 Suran et a1.: TWo Terminal Analizers and Synthesis of 10/1961 Wedig 330--20 Junction Transistor Multivibrators, IRE Transactions,12/1961 Horton et a1 328241 March 1956.

2/1962 Erath 330-20 5 ARTHUR GAUSS, Primary Examiner.

HERMAN K. SAALBACH, JOHN W. HUCKERT,

Examiners.

FOREIGN PATENTS 4/1956 France. 12/1960 Germany.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,223,849 December 14, 1965 Carl D. Todd It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 2, lines 4 and 5, for "expaining" read explaining column 5, line7, strike out "and", and insert instead a comma; column 6, line 35,strike out "that current flows in a direction opposite the source 88so"; column 8, linr l5 and 16, the formula should appear as shown belowinstead of as in the patent:

same column 8, lines 55 and 56, the formula should appear as shown belowinstead of as in the patent:

+v V (1+h )R R2: BB2 EB 1 F13 11 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A CIRCUIT FOR PROVIDING A CRRENT-VOLTAGE CHARACTERISTIC HAVING ALINEAR NEGATIVE RESISTANCE PORTION OF PRESELECTED MAGNITDE BETWEEN FIRSTAND SECOND POSITIVE RESISTANCE PORTIONS COMPRISING: FIRST AND SECONDTERMINALS BETWEEN WHICH SAID CURRENT-VOLTAGE CHARACTERISTIC IS PROVIDED;A SORCE OF POTENTIAL HAVING ONE OF ITS TERMINALS CONNECTED TO SAIDSECOND TERMINAL; FIRST AND SECOND TRANSISTORS OF OPPOSITE CONDCTIVITYTYPES EACH HAVING AN EMITTER, A COLLECTOR AND A BASE; THE COLLECTOR ANDBASE OF SAID FIRST TRANSISTOR BEING DIRECTLY CONNECTED TO SAID FIRST ANDSECOND TERMINALS, RESPECTIVELY; THE EMITTER OF SAID FIRST TRANSISTORBEING DIRECTLY CONNECTED TO THE EMITTER OF SAID SECOND TRANSISTOR; THECOLLECTOR OF SAID SECOND TRANSISTOR BEING DIRECTLY CONNECTED TO THEOTHER TERMINAL OF SAID SOURCE OF POTENTIAL; RESISTANCE MEANS CONNECTEDBETWEEN THE BASE OF SAID SECOND TRANSISTOR AND SAID OTHER TERMINAL OFSAID SOURCE OF POTENTIAL FOR PROVIDING A SUBSTANTIALLY CONSTANT CURRENTFLOW THEREBETWEEN; AND A FEEDBACK RESISTOR CONNECTED BETWEEN THECOLLECTOR OF SAID FIRST TRANSISTOR AND THE BASE OF SAID SECONDTRANSISTOR FOR DECREASING THE BASE CURRENT OF SAID SECOND TRANSISTOR INRESPONSE TO AN INCREASE IN VOLTAGE BETWEEN SAID FIRST AND SECONDTERMINALS; SAID FEEDBACK RESISTOR HAVING A RESISTANCE VALUE EQUAL INMAGNITUDE TO THE COMMON EMITTER CURRENT GAIN OF SAID SECOND TRANSISTORTIMES SAID PRESELECTED MAGNITUDE OF SAID LINEAR NEGATIVE RESISTANCE.